Method and apparatus for determining direction

ABSTRACT

A map is provided for indicating the direction of a fixed point (S), such as a sub-satellite point, from any point (P) within a specified area which is a substantial fraction of the earth&#39;s surface. The map projection is such that the actual direction to the fixed point (S) is the same as the direction shown on the map, when the map is aligned with a reference direction such as magnetic north. Optionally, the distance to the fixed point on the map represents the elevation of the satellite. Apparatus for using the map includes a holder (10) for the map and for a magnetic compass (18). A scale (22) which rotates about the fixed point on the map has a marker (32) which is positioned over the user&#39;s location on the map. The azimuthal direction of the satellite is indicated by an arrow (28) on the scale, while the elevation of the satellite is read off the scale by means of a pointer (34) located on the fixed point on the map.

TECHNICAL FIELD

The present invention relates to a method and an apparatus fordetermining the direction of a defined position from a variableposition. The present invention is particularly directed towardsdetermination of the direction that an antenna for a portablecommunications terminal (the variable position) must be pointed to finda communications satellite (the defined position).

In telecommunications, a signal may be transmitted using microwaves orover a cable. As microwaves are easily absorbed by solid objects, theuse of microwaves in telecommunications requires that the transmitterand receiver be in line of sight. Communications satellites provide amethod of achieving line of sight communication over long distances.Communication between the satellite and terrestrial communicationnetworks is provided via a land earth station which lies within thecoverage area of the satellite.

In remote areas, where telephone services are either inadequate orunavailable, portable telephones and data terminals which directlyaccess a communications satellite may be used, by means of a satellitecommunications service such as the InmarsatM^(SM) service. Suchsatellite telephones, also known as Mobile Earth Stations (MES) arebeing used by news agencies, U.N. relief agencies and other groups whichoperate in remote areas. New models of MES may be as small as a largebriefcase.

One particular feature of an MES, as compared with cellular telephones,is that the antenna is not small and omni-directional but, instead, mustbe pointed towards the satellite before the MES can be used. The antennais relatively large, about 30 cm in diameter and typically has abeamwidth of 15° to 20°. Thus, a user of an MES must be able todetermine the direction of the satellite from his location sufficientlyaccurately to pick up a signal from which the antenna can be directedmore accurately by optimising the signal--he must be able to determineboth the compass direction to the satellite (the azimuthal component)and the elevation of the satellite in the sky. It is also useful to beable to make at least a rough determination, before the MES is set up,of the direction of the satellite from the user's general location toensure that there are no solid objects, such as buildings or trees,blocking the line of sight between the satellite and the user'slocation.

BACKGROUND ART

Various methods have been used for determining the direction to asatellite from a variable location. It is known to use a computer orprocessor to calculate the required elevation and azimuth to a selectedsatellite from data regarding the location of the user. Such data may beentered into the computer as the user's latitude and longitude read froma world map or by indicating the user's location using a cursor in anelectronic display showing a world map. Alternatively, the user can makeuse of a map, such as that shown in FIG. 1. The map is of a standardprojection but the map grid does not show the normal lines of latitudeand longitude. Instead, the map grid is a curvilinear grid showing linesof azimuth and elevation for a particular satellite so that the user candetermine the correct azimuth and elevation for his location by findingthe location on the map. In all these methods, the user then employs acompass to point the antenna in the required azimuthal direction andadjusts the angle of the antenna to set the elevational component of thedirection to the satellite.

Disclosure of the Invention

According to a first aspect of the present invention, there is provideda method of determining the direction of a defined position from avariable position within a specified area, the specified area includingpositions which are separated from said defined position by a number ofdegrees of latitude or longitude greater than a predetermined limit,using an arrangement comprising a map of said specified area and a mapdefined point positioned relative to said map to represent said definedposition relative to said specified area, said map having a mapreference direction by which the azimuthal orientation of the map isdetermined in use, said map having a projection in which map points onthe map represent positions in reality such that:

i) azimuthal position-reference directions from each position in realityto a single reference position are representable by map point-referencedirections on the map which are parallel to or aligned with said mapreference direction to within said predetermined limit;

ii) a map bearing on the map of a map point-defined direction from a mappoint on the map to said map defined point relative to said mapreference direction equals within said predetermined limit a bearing inreality of an azimuthal position-defined direction from a position inreality to said defined position in reality relative to an azimuthalposition-reference direction from said position in reality to saidreference position in reality;

the method comprising the step of determining the azimuthal component ofsaid direction of said defined position from said variable position by

a) orienting said map so that said map reference direction is alignedwith the azimuthal direction from said variable position in reality tosaid single reference position;

b) locating a map variable point on the map representing said variableposition in reality; and

c) determining a map variable-defined direction from said map variablepoint to said map defined point which represents said azimuthalcomponent of said direction of said defined position from said variableposition.

The present invention relates to the determination of direction betweenpositions which are separated from one another by at least a certain isdistance, e.g. the global distances involved when determining thedirection of a satellite from a position within the coverage region ofthe satellite. It is well known that the projection of the curvedsurface of the earth onto a planar surface will involve some distortionof scale, bearing, shape or area. This distortion becomes significantfor large distances and may be too great to be tolerated for certainapplications.

In arriving at the present invention, the inventor has realised that theaction of determining the direction of a defined position from avariable position over a global area, particularly in the context ofdetermining the direction an antenna should be pointed to communicatewith a satellite, need not be as complicated as has been the case in theprior art. The prior art operation has required that informationregarding the azimuthal direction be derived from a computer or a map.It is then necessary to transfer this information to the context ofdetermining the azimuthal direction in reality from the user's locationby using a compass. In contrast, the inventor has appreciated that theazimuthal direction can be determined in reality and visually by using amap of the correct projection. When such a map is orientated withreality, the direction between points representing the user's locationand a defined position on the map will correspond to the requiredazimuthal direction in reality of the defined position from the user'slocation and so may be seen visually from the map without the need forthe information to be processed further in any other way.

Where the method uses a magnetic compass, the single reference positionis magnetic north. Thus, the map projection accommodates both errors dueto the distortion involved in projecting a curved surface onto a planarsurface and also the distortion caused by the fact that magnetic northis not the same as true north.

Advantageously, the map has a projection such that map points which areequidistant from said map defined point represent positions in realitywhich are equidistant from said defined position in reality, the methodincluding the further step of determining the elevational component ofsaid direction of said fixed position from said variable position bydetermining the separation of said map variable point from said mapfixed point, said separation representing said elevational component.The same map may therefore be used to determine both the azimuthalcomponent and the elevational component of a direction from a variableposition to a defined position, such as the position of a satellite.

It will be appreciated that this advantageous feature is particularlyapplicable to a method of determining the direction of a satellite froma variable position in which the specified area is the coverage regionof the satellite. Thus, the first aspect of the present invention alsoprovides a method of setting up a satellite antenna.

A second aspect of the present invention provides an apparatus fordetermining the direction of a defined position from a variable positionwithin a specified area, the apparatus comprising:

compass means;

map means for providing a map, said map means including compass holdingmeans for holding the compass means relative to said map means.

Thus, if the user orientates the map with reality by using the compassmeans, then the direction between the user's location and the definedposition on the map will correspond to the required azimuthal directionof the defined position from the user's location. The map means iseffective to provide a map in a flat position.

Determination of the required azimuthal direction of the definedposition from the user's location is sufficient when the definedposition is a terrestrial location. One such defined position is Meccawhich Muslims are required to face when they pray.

If the compass means includes a magnetic compass, a modified map may beused in which points on the map have been moved to compensate formagnetic declination.

It is envisaged that a map of the required projection may form at leastpart of the map means. Alternatively, or in addition, the map means maycomprise map holding means for holding one or more maps in a flatposition. The map holding means may be arranged for releasably holdingthe map so that the same structure may be used for different mapsdepending on the location of the user and the satellite to be used.

Advantageously, the apparatus further comprises straight line means forrepresenting a straight line extending at least from a fixed location toa different location on the map, said straight line means and said mapmeans preferably being rotatable relative to one another about saidfixed location. When the apparatus is arranged with the map definedpoint coincident with said fixed location and the straight linerepresented extends from said fixed location to the map pointrepresenting the required variable position, this facilitatesdetermination of the azimuthal direction.

Preferably, said straight line means includes marker means located onsaid straight line for marking out another location on the map, saidmarker means advantageously comprising magnifying means so that thelocation of a map point representing said variable position on the mapcan be easily found. For this reason, said marker means areadvantageously movable along said straight line relative to the fixedlocation on the map.

Conveniently, said straight line means includes a straight line markeither aligned with said straight line or extending parallel to saidstraight line from which the require azimuthal direction may be easilydetermined.

The map may further include a direction indicator for indicating acompass direction of the map, such as north, relative to the compassmeans so that the compass needle of the compass means can be easilyaligned with the compass direction to ensure that the map is orientatedwith reality.

It is further envisaged that the compass means may be suppliedseparately from the remainder of the apparatus. Accordingly, a thirdaspect of the present invention provides an apparatus for determiningthe direction of a defined position from a variable position within aspecified area, the apparatus comprising:

map means for providing a map, the map means including straight linemeans for representing a straight line extending at least from a fixedlocation to a different location. The map means are effective to providea map in a flat position.

An advantage of the present invention lies in its simplicity so that itcan be easily used in remote areas with a portable satellite telephoneor other means for communicating with a communications satellite.

Accordingly, a fourth aspect of the present invention provides thecombination of means for communicating with a communications satelliteand an apparatus according to any one of the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, and with reference to the accompanying drawings, in which:

FIG. 1 shows a prior art map of the Atlantic Ocean region with a mapgrid of lines of azimuth and elevation;

FIGS. 2a, 2b, 2c, 2d, 2e and 3a, 3b, 3c, 3d, 3e illustrate anexplanation of the production of a map having a map projection suitablefor use with embodiments of the present invention;

FIG. 4 shows a first embodiment of an apparatus provided in accordancewith the present invention;

FIGS. 5a, 5b, 5c and 5d show components of the apparatus of FIG. 4;

FIGS. 6a, 6b, 6c and 6d show a second embodiment of an apparatusprovided in accordance with the present invention; and

FIG. 7 shows an MES communicating with a land earth station by means ofa directional antenna pointed at a satellite.

MODES OF CARRYING OUT THE INVENTION

FIGS. 2 and 3 illustrate an explanation of how a standard map projectionmay be modified to derive the novel map projection of the presentinvention.

FIG. 2a represents a standard Orthographic projection. This example iscentred, at 0 deg latitude, 15 deg West Longitude, and the part of theglobe shown represents the "region of interest", namely the coverageregion of an Inmarsat AOR-E satellite. The AOR-E satellite is ingeosynchronous orbit approximately at 15 deg West Longitude.

FIG. 2b shows the same map, and indicates a point labelled "S", in thiscase the sub-satellite point, which represents the point toward whichthe user wants to find the direction, or "azimuth". The user may want tobe able to find the distance or elevation as well. Also in the secondfigure is point "P", an arbitrary point of the map, which is to bereplotted in the final map projection. Between points P and S is arc C,which represents the "great circle" route from P to S, i.e. the route ofshortest distance from P to S along the globe surface.

Arc C between P and S has two properties of interest. One is the length,which is roughly proportional to the great circle distance from P to S.Consequently, when S represents a sub-satellite point, the arc C isapproximately proportional to the elevation of the satellite to anobserver at P. The second property of interest is angle A between arc Cand a line running north, shown in the expanded area in FIG. 2c. In thisexpanded view, the direction toward north, for an observer at point P,is as shown. One might incorrectly position the local north arrowvertically, if one did not consider that a flat map cannot represent aspherical globe adequately. As an assurance, note that the indication oflocal north is always tangent to a line of longitude at point P. Angle Ais that angle which lies in a plane tangent to the globe's surface atpoint P; this plane is not coincident with the plane of the paper onwhich the map is drawn, and so it must be calculated and cannot bemeasured directly from the plot. For example, note that on a sphericalglobe, lines of latitude are always perpendicular to lines oflongitude--but in the expanded view shown here they appear not to be so.

In FIG. 2d, a circle has been drawn, in which the new projection will beplotted. Point S has been conveniently placed in the centre of thecircle. Point P has been plotted such that the length of line segment PSis proportional to the length of C, and angle A as drawn on paper equalsangle A as calculated in FIG. 2c. Note that in the new projection inFIG. 2d, North is always a vertical line, because the map will beoriented with North at the top, whenever a user is physically located atsome point P.

This procedure of translating point P from its position in the map inFIG. 2a to the new position in the projection of FIG. 2d is repeated forall points on the map within the desired map region, resulting in FIG.2e.

It will be appreciated that the novel map projection may, in fact, beprepared using an appropriate computer program and a database of pointsof longitude and latitude.

As indicated, the map projection of FIG. 2e has a map referencedirection, by which the azimuthal orientation of the map is determinedin use, pointing towards North. Other reference directions may be chosendepending on how the map is to be orientated in use. In particular, itwill be appreciated that conventionally used magnetic compasses pointtowards magnetic north pole rather than true north pole. This magneticdeclination varies at different points on the earth's surface. Forexample, in Iceland the compass error due to magnetic declination isabout 20°. A map may therefore be modified, as shown in FIGS. 3a, 3b,3c, 3d and 3e to compensate for this magnetic declination. FIG. 3a showsa map of the projection of FIG. 2c. An arrow C' shows the correctdirection from Eastern Iceland (P') to the map defined point S',(representing the point toward which the user wants to find theazimuthal direction) if the map is orientated towards true north.However, if a magnetic compass is used, the actual orientation of themap will be as shown in FIG. 3b and so there will be an error of 20° inthe azimuthal component. To compensate for this magnetic declination,the map may be redrawn, as shown in FIG. 3c, such that Eastern Iceland(P") is moved 20° in an arc around the defined point S'. Then, as shownin FIG. 3d, the arrow between the user's location in Eastern Iceland(P") and the defined point S' will be pointing in the correct direction.

As the magnetic declination will vary from place-to-place on the earth,it will be necessary to repeat this procedure for every point ofinterest on the map. The arc around the fixed point through which eachlocation must be moved is equal and opposite to the direction of thelocal magnetic declination. In practice, such modifications to a map tocompensate for magnetic declination could be prepared by a draughtsman,or with the aid of a computer, with the help of a map showing globalmagnetic declination, such as Chart 5374 published by the BritishGeological Survey. A map projection prepared in this way is shown inFIG. 3e.

It will be appreciated that the projection of FIG. 3e may be sodistorted relative to standard projections that the user may finddifficulty in recognizing locations on the map. Thus, the projection ofFIG. 3e may be further modified within certain limits for artistic andusability reasons. For use with a satellite antenna, the map bearings ofthe projection should be equal to the required bearings in realitywithin the limit of the beamwidth of the antenna, typically 15 to 20°.It will be appreciated that the area of the map covers a number ofdegrees of latitude or longitude which is greater than the limit set onthe accuracy of the bearings by the use of the map. For maps in whichall points of the map are separated from the point of interest by anumber of degrees of longitude or latitude less than the accuracy of themap bearings, the need to use a map of the novel projection is lesscritical.

In a general form, an embodiment of an apparatus of the presentinvention may consist of a board on which is fixed a map and a compass.An arrow is fixed on the map in such a way that it rotates about a mapdefined point representing the defined position relative to the area ofthe map. It is not essential that the map defined point be within themap although the map defined point is typically at the centre of themap. The point of the arrow is at the map defined point on the map andthe tail of the arrow is at the edge of the map so that the length ofthe arrow equals the radius of the map. Because both the map and thecompass are held by the board, re-orientation of the board will affectthe orientation of the map as indicated by the compass. Using thecompass, the user therefore positions the north side of the map towardsnorth in reality so that the orientation of the map conforms withreality. The arrow is then rotated such that it lies on top of a pointrepresenting the user's current position on the map. Because the map hasbeen oriented to conform with reality, the arrow now points in theazimuth direction towards the sub-satellite position. The antenna of thesatellite telephone can therefore easily be pointed in the azimuthdirection of the sub-satellite point by aligning the antenna with thearrow.

It has further been appreciated that the separation of the map definedpoint from the point representing the user's current location on the mapis, using this projection, proportional to the elevation of thesatellite from the user's current location. The arrow may therefore becalibrated with a series of numbers ranging from 0 at the arrow's pointto 90 at the arrow's tail. If the arrow is allowed to slide forward andbackward along its length, its direction will not change but its radialposition can be adjusted so that the tail of the arrow lies on theuser's current location. The number written on the arrow which lies atthe pivot is the satellite elevation.

It is further envisaged that the present invention may be applied toother circumstances in which a person needs to be able to determine thedirection from his current location to a defined position in the world.One such fixed position is Mecca as Muslims are required to face towardsMecca when they pray. As Mecca is a terrestrial location, elevation isnot required so the arrow must be able to rotate but does not need to beable to slide. The map coverage required will obviously depend on theareas in which the user will find himself. The implementation ispreferably suitable for permanently fixing onto a prayer rug.

FIG. 4 shows a first embodiment of the present invention as used fordetermining the direction of a satellite from a variable position. Theapparatus comprises a holder or envelope 10 having a slot 12 into whicha map may be slid. As the holder 10 does not fixedly hold the map, thestructure may be used with different maps depending on the location ofthe user and the satellite which the user intends to use. Conveniently,the holder is made of plastics material. At least the front face 14 ofthe holder is transparent so that the map is visible from outside. Asshown in FIG. 5a, the holder 10 further includes a hole or inset 16 forholding a compass 18. A hole 20 is provided at a fixed location ontowhich an indicator 22 can be mounted using a pivot structure 24.

The indicator 22, as shown in FIG. 5b, comprises a plastic rectangularstructure which is held on the holder 10 by the pivot 24. The indicator22 includes a straight slot 26, through which the pivot 24 extends, suchthat the indicator 22 can be rotated about the pivot and also slidbackward and forward relative to the pivot along the slot. The indicator22 also includes an arrow 28 to indicate azimuth direction and iscalibrated, as at 30, to indicate elevation. A lens 32 is provided atone end of the slot 26 to assist in determining the point representingthe user's exact position on the map.

The pivot structure 24 is shown in greater detail in FIG. 5c. The pivotstructure includes a pointer 34, effective to define the fixed positionon the map, and used to point to the calibrations 30 on the indicator 22which refer to elevation. Beneath the pointer 34 is a tab 36 relative towhich the indicator 22 can slide. Beneath the tab 36 (not visible inFIG. 5c) is a plug by which the pivot structure 24 is securely mountedin the holder 10. The pivot structure therefore fastens the indicator 22to the holder 10 but allows the indicator 22 to both rotate and slide.

In the embodiment of FIG. 4, the arrow 28 is drawn to lie parallel tothe straight slot 26. The indicator 22 is effective to represent astraight line from the map defined point on the map to another point onthe map because of the lens through which the point representing theuser's exact location on the map can be identified. It is envisaged thatthe indicator 22 may be effective to represent a straight line even ifno straight line is actually marked on the indicator. For example,dotted lines may be used to represent a straight line. Alternatively,the direction of a straight line can be uniquely defined by two points,one of which is the map defined point and the other of which assists indefining the user's current location. The lens 32 advantageouslymagnifies the map for easier identification of the point representingthe user's current location on the map.

FIG. 5d shows a sheet 38 for insertion into the holder 10. The sheet 38is preferably dimensioned so that it can be easily slid into the holder10 but is not movable within the holder 10. The circle referenced 40indicates the position of the printed map and the point 42 indicates themap defined point or sub-satellite point. This point 42 is positionedimmediately below the fixed location of the hole 20 and so correspondsin position to the pivot structure 24. Also printed on the sheet 38 is aline 44 indicating the direction north for the map 40. The line 44 is sopositioned on the sheet 38 that it will be in the right positionrelative to the compass 18 in the assembled structure. Thus, when themap is orientated so that the compass needle, pointing north, is alignedwith the line 44, then the map also will be orientated with its northside towards north. Thus, the apparatus provides a simple structure bywhich a map may be correctly orientated with reality so that a directionindicated by the map points to the direction in reality.

FIG. 6a shows a second embodiment of an apparatus according to thepresent invention in which the apparatus is incorporated into theequipment which is to be pointed towards a defined position. Theapparatus 50 is provided on an MES 52 having a directional antenna 54. Acompass 56 is mounted at the centre of a map 58 of the requiredprojection. north is indicated on the map 58, as by a line 60, so thatnorth as indicated by the compass can be easily aligned with the northof the map 58. A strip 62 is mounted to be slidable in a slot of the MES52 and aligned with or parallel to the antenna direction. Means (notshown) are provided to enable the map 58 and the strip 62 to be rotatedrelative to one another. Also shown in FIG. 6a, to assist a descriptionof the operation of the apparatus, is a compass star 64 to indicatenorth in reality.

In FIG. 6b, the map 58 has been rotated relative to the MES 52, as awhole, and the strip 62 such that the point representing the user'sposition (in this example, London) is over the strip 62.

In FIG. 6c, the MES 52, as a whole, has been rotated until the north ofthe map, as indicated by the line 60, is aligned with the compass north.As the strip 62 is aligned with the antenna 54 and with the pointsrepresenting the defined position and the user's position on the map,the antenna is therefore pointing in the correct azimuthal directiontowards the defined position of the satellite.

In FIG. 6c, the strip 62 is slid until its upper end is positioned atthe point representing London on the user's map. The strip 62 iscalibrated to indicate the elevation of the satellite. Thus, therequired elevation can be read off the strip 62 and so the elevation ofthe directional antenna 54 can be adjusted.

In FIG. 7, the azimuthal direction and elevation has been arranged sothat the directional antenna 54 is pointing at the satellite, indicatedby reference 66. The user of the MES 52 then initiates a call via thesatellite 66 to a land earth station (LES) 68, which provides a gatewayto a terrestrial telephone network 70. The MES 52 may have data or faxcommunication facilities, as well as or instead of audio communicationfacilities.

In the embodiments shown, the map is releasably mounted in the apparatusso that the same apparatus may be used for different maps depending onthe location of the user and the satellite to be used. Alternatively,one or more maps may be printed onto a card to which a compass and/or astraight line marker may be affixed.

Map projections are also envisaged where the required defined point is aterrestrial location such that there is no elevational component. Insuch map projections, there may be regions of the map which may be movedcloser together on the map without losing azimuthal information. Themodified map is smaller than the map of FIGS. 2 and 3. Because of themodification of the map, the elevation information has been lost but theazimuthal information has been retained.

Industrial Applicability

The method of the present invention may be applied in the field oftelecommunications, while the apparatus of the present invention may bemanufactured industrially.

I claim:
 1. A method of determining the direction of a defined positionfrom a variable position within a specified area, the specified areaincluding positions which are separated from said defined position by anumber of degrees of latitude or longitude greater than a predeterminedlimit, using an arrangement comprising a map of said specified area anda map defined point positioned relative to said map to represent saiddefined position relative to said specified area, said map having a mapreference direction by which the azimuthal orientation of the map isdetermined in use, said map having a planar projection in which mappoints on the map represent positions in reality such that:i) azimuthalposition-reference directions from each position in reality to a singlereference position are representable by map point-reference directionson the map which are parallel to or aligned with said map referencedirection to within said predetermined limit; ii) a map bearing on themap of a map point-defined direction from a map point on the map to saidmap defined point relative to said map reference direction equals withinsaid predetermined limit a bearing in reality of an azimuthalposition-defined direction from a position in reality to said definedposition in reality relative to an azimuthal position-referencedirection from said position in reality to said reference position inreality; the method comprising the step of determining the azimuthalcomponent of said direction of said defined position from said variableposition by a) orienting said map so that said map reference directionis aligned with the azimuthal direction from said variable position inreality to said single reference position; b) locating a map variablepoint on the map representing said variable position in reality; and c)determining a map variable-defined direction from said map variablepoint to said map defined point which represents said azimuthalcomponent of said direction of said defined position from said variableposition.
 2. A method according to claim 1, wherein said map definedpoint is within said specified area.
 3. A method according to claims 1or 2, wherein the step of orienting the map uses a magnetic compass andsaid single reference position is magnetic north.
 4. A method accordingto claim 1, wherein said specified area covers a number of degrees oflatitude or longitude which is greater than said predetermined limit. 5.A method according to claim 1, said map having a projection such thatmap points which are equidistant from said map defined point representpositions in reality which are equidistant from said defined position inreality, the method including the further step of determining theelevational component of said direction of said defined position fromsaid variable position by determining the separation of said mapvariable point from said map defined point, said separation representingsaid elevational component.
 6. A method of determining the direction ofa satellite from a variable position using the method of claim 5,wherein said specified area is the coverage region of the satellite. 7.A method of setting up a satellite antenna, comprising the method ofclaim 6 and the further step of directing the antenna in said mapvariable-defined direction.
 8. A method of communication via satellite,comprising the method of claim 7 and the further step of sending signalsto said satellite through said antenna.
 9. An apparatus for determiningthe direction of a defined position from a variable position within aspecified area, the apparatus comprising:a map of said specified area,the specified area including positions which are separated from saiddefined position by a number of degrees of latitude or longitude greaterthan a predetermined limit, and a map defined point positioned relativeto said map to represent said defined position relative to saidspecified area, said map having a map reference direction by which theazimuthal orientation of the map is determined in use, said map having aplanar projection in which map points on the map represent positions inreality such that: i) azimuthal position-reference directions from eachposition in reality to a single reference position are representable bymap point-reference directions on the map which are parallel to oraligned with said map reference direction within said predeterminedlimit; ii) a map bearing on the map of a map point-defined directionfrom a map point on the map to said map defined point relative to saidmap reference direction is equal within said predetermined limit to abearing on reality of an azimuthal position-defined direction from aposition in reality to said defined position in reality relative to anazimuthal position-reference direction from said position in reality tosaid reference position in reality; and means for indicating theazimuthal orientation of the map.
 10. An apparatus according to claim 9,further comprising straight line representing means for representing astraight line extending at least from said map defined point to adifferent location, wherein said straight line representing means andsaid map are rotatable relative to one another.
 11. An apparatusaccording to claim 10, wherein said straight line representing meansincludes a marker located on said straight line for marking saiddifferent location.
 12. An apparatus according to claim 11, wherein saidmarker comprises magnifying means for magnifying an area of the maparound said different location.
 13. An apparatus according to claims 11or 12, wherein said marker is movable along said straight line relativeto said map defined point.
 14. An apparatus according to claim 10,wherein said straight line representing means includes a straight linemark.
 15. An apparatus according to claim 14, wherein said straight linemark extends parallel to said straight line.
 16. An apparatus accordingto claim 10, wherein said map has a projection such that map pointswhich are equidistant from said map defined point represent positions inreality which are equidistant from said defined position in reality, thestraight line representing means being calibrated to indicate saidelevational component.
 17. An apparatus according to claim 9, whereinsaid map defined point is within said specified area.
 18. An apparatusaccording to claim 9, wherein said specified area covers a number ofdegrees of latitude or longitude which is greater than saidpredetermined limit.
 19. An apparatus according to claim 9, wherein saidsingle reference position is magnetic north and said means forindicating the azimuthal orientation of the map comprises a magneticcompass azimuthally fixed relative to the map.
 20. An apparatusaccording to claim 9, including a map holder for releasably holding themap.
 21. A satellite earth station including a directional antenna andan apparatus according to claims 9 or
 10. 22. A satellite earth stationincluding a directional antenna and an apparatus according to claim 10,wherein said directional antenna is aligned with or parallel to saidstraight line.
 23. A map of a specified area, the specified areaincluding positions which are separated from a defined position by anumber of degrees of latitude or longitude greater than a predeterminedlimit, and a map defined point positioned relative to said map torepresent said defined position relative to said specified area, saidmap having a map reference direction by which the azimuthal orientationof the map is determined in use, said map having a planar projection inwhich map points on the map represent positions in reality such that:i)azimuthal position-reference directions from each position in reality toa single reference position are representable by map point-referencedirections on the map which are parallel to or aligned with said mapreference direction within said predetermined limit; and ii) a mapbearing on the map of a map point-defined direction from a map point onthe map to said map defined point relative to said map referencedirection is equal to within a predetermined limit to a bearing inreality of an azimuthal position-defined direction from a position inreality to said defined position in reality relative to an azimuthalposition-reference direction from said position in reality to saidreference position in reality.
 24. A map according to claim 23, whereinsaid projection is such that map points which are equidistant from saidmap defined point represent positions in reality which are equidistantfrom said defined position in reality.
 25. A map according to claim 23or 24, wherein said map defined point is within said specified area. 26.A map according to claim 23, wherein said specified area covers a numberof degrees of latitude or longitude which is greater than saidpredetermined limit.
 27. A map according to claim 23, wherein saidsingle reference position is magnetic north.
 28. A satellite earthstation including a directional antenna and a map according to claim 23.